Mechanism of cellular damage
The core physical principle of cryoablation is based on the Joule-Thomson effect. The delivery of liquid argon into a system of thin probes is accompanied by its transition into a gaseous state. This phase shift causes rapid heat absorption, resulting in tissue temperatures directly adjacent to the instrument dropping to extreme values ranging from -150°C to -170°C.
To ensure guaranteed necrosis throughout the target volume, a double freeze-thaw cycle is always used in practice. The pathogenesis of tumor tissue destruction is multifactorial:
- Intracellular crystallization: the formation of ice crystals inside the cell leads to the physical rupture of membranes and vital organelles.
- Extracellular ice formation: fluid freezes in the interstitium, causing severe osmotic dehydration and cell shrinkage.
- Vascular factor: cold exposure severely damages the endothelium of the microvasculature, provoking microthrombosis and ischemia in the treatment zone.
Technique and image guidance
The safety and clinical success of the procedure largely depend on the quality of image guidance (Image Guidance). At the pre-procedure planning stage, it is critically important to maximize tumor visibility (conspicuity). Ultrasound and advanced high-contrast diagnostic imaging methods are carefully utilized for this purpose.
During the procedure itself, a group of several argon cryoprobes (usually 3 to 7) is percutaneously inserted into the tumor tissue. The instruments are placed strictly parallel, stepping back about 10 mm from the edge of the neoplasm and maintaining a distance of 15–20 mm from each other.
A unique feature of cryoablation is the formation of a dense physical "ice ball" (iceball). Unlike many other destruction technologies, the freezing zone grows extremely predictably and is clearly contrasted on ultrasound, CT, and MRI. This makes it possible to monitor the boundaries of the therapeutic zone in real time and avoid damage to adjacent organs.
Alternative ablation methods
In radiation surgery and interventional radiology, besides low temperatures, other physical and chemical principles of tissue impact are actively used:
- Focused ultrasound (Focused Ultrasound). A completely non-invasive technique. Sound energy with a frequency of about 1 MHz is focused using an extracorporeal acoustic lens. Small ovoid destruction foci (grain-sized, 12 × 3 mm) are formed and sequentially overlapped. The method has limitations: energy rapidly attenuates when passing through gas or bone. Due to the need for respiratory synchronization and complex motion tracking, it is most often used on stationary organs (uterus, prostate gland).
- Irreversible electroporation (Irreversible Electroporation - IRE). A non-thermal method. Millisecond direct current pulses are applied between monopolar probes (or via a single bipolar probe). This leads to irreversible poration of cell membranes. The method spares blood vessels and collagen, but causes strong muscle contractions (general anesthesia with muscle relaxants is required) and requires strict ECG synchronization to prevent cardiac arrhythmias.
- Interstitial laser photocoagulation (Interstitial Laser Photocoagulation - ILP). Laser light (often from an Nd:YAG source) is absorbed by tissue chromophores, generating heat. It forms small coagulation zones (about 10 mm). For a clinically significant volume, bulky systems and beam splitters (beam splitter) are required.
- Chemical ablation. Direct infusion of ethanol or acetic acid causing protein denaturation. Previously widely used for treating small hepatomas. Due to the uneven distribution of the agent in large nodes, the method is being replaced today by thermal technologies, remaining relevant mainly for healthcare systems with limited budgets (where multi-needle probes are used).
For quick memorization of the pathogenesis of cryoablation, use the rule "ICE": I — Ice (intracellular organelle rupture), C — Cellular dehydration (due to extracellular freezing), E — Endothelium (microvascular damage and ischemia).
Class and exam questions
Why are several probes used simultaneously in cryoablation instead of one?
The effective freezing zone around a single instrument is limited. To completely cover the tumor, probes are grouped (3–7 pieces), creating a single volumetric destruction focus—an ice ball.
What is the main advantage of cryoablation when working near the kidneys?
It does not destroy collagen fibers. This allows the method to be safely used near the pelvicalyceal system without fear of perforation and irreversible damage to its framework.
Why is ECG synchronization necessary during irreversible electroporation (IRE)?
The method uses powerful millisecond direct current discharges that can affect the cardiac conduction system. Strict synchronization of pulses with the heart rhythm prevents the development of fatal arrhythmias.
How does irreversible electroporation differ from reversible?
Reversible electroporation temporarily porates the membrane and is used for gene transfection. Irreversible uses current slightly longer, leading to fatal membrane destruction and controlled cell death.
What else to study on this topic
- Motion tracking technologies during focused ultrasound exposure
- Principles of operation of extracorporeal acoustic lenses
- Mechanisms of reversible and irreversible membrane poration
- The role of beam splitters in interstitial laser thermotherapy
- Algorithms for pre-procedural enhancement of tumor conspicuity
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